When a plane crashes, it can take many months or years to find the black box that can provide clues as to what happened. Just what are these devices, how do they work, and why can they be so hard to find? We contacted the recorders division of the National Transportation and Safety Board to find out.

What is a "black box"?

An airplane actually has two black boxes: a flight data recorder, which stores information on specific parameters such as flight control and engine performance, and a cockpit voice recorder, which records background sound and conversations between crew members and air traffic control. The "black box" isn't actually black at all. It is painted bright orange in order for it to be more easily spotted in wreckage.

How is the black box designed?

To protect the stack of memory boards that store information, black boxes are wrapped in a thin layer of aluminum and a 1-inch layer of high-temperature insulation, and then encased in a corrosion-resistant stainless steel or titanium shell. The black box must be able to withstand an acceleration of 3,400 Gs (3,400 times the force of gravity), which equals an impact velocity of about 310 mph. It must also survive flames up to 2,000 degrees F for one hour, and the beacon should be able to emit a signal once per second while submersed in 20,000 feet of saltwater for 30 days. The beacon cannot be heard by the human ear but can be easily be detected by sonar. The beacon is battery-powered, with a shelf life of up to six years.

Why does it take so long to find them?

Search teams must be in range of the beacon, about 15 miles. However, a beacon could be snapped off during a high-impact crash.If a crash occurred at sea, Hamilton says finding the black box has "less to do with how deep the water is. What is more relevant is how soon debris is found and how close it is to the point of impact." The more time it takes to find debris, the more time winds and currents have to carry it away. There are other factors, too, like how long it takes to mobilize search and rescue teams, and how sure authorities are of where the plane crashed. By the time debris is found, it could be miles away from the initial impact site. Underwater terrain such as deep trenches could also affect how easily the pinging can be detected.

Where is the black box located?

The black box is typically installed in the tail of the plane, which is usually the last portion of the plane to impact.

How much data can a black box store?

A flight data recorder is required to store a minimum of 25 hours of flight information. A cockpit voice recorder is required to record a minimum of 2 hours of audio information.

What would it take to destroy a black box?

"It is extremely rare for a black box to be destroyed," says Scott Hamilton, director of Leeham Co., an aviation consulting company. "Black boxes have traditionally outperformed their design." Hamilton says he cannot think of a single case in which both devices have been damaged to the point to where there is no useful data. "It would take a concentrated fire beyond its design strength, or an impact so high that it would be beyond what it could withstand."

Has a black box ever been destroyed?

There are a handful of cases in which black boxes have not been recovered, and a couple of cases in which the flight data recorder was found but not the cockpit voice recorder, or vice versa. Rarely, a recorder is recovered but blank or too damaged to read. Could black boxes transmit information out rather than store it in, so that search teams don't have to go into dangerous terrain to find them? "The viability of that technology is very good," aviation security consultant Chris Yates told Renee Montagne on Morning Edition. "Of course the big question is whether the airline industry that often bleats on out about the fact that it is constantly losing money hand over fist will want to invest in that technology."